MOLLUSCAN EVIDENCE BEARING ON CENOZOIC WARM UPWELLING OFF SOUTHERN PERU

The Peruvian margin may have experienced long periods of elevated sea-surface temperatures (SSTs) and ‘warm coastal upwelling' during the Cenozoic. These ‘El Niño-like conditions' would have contrasted sharply with Recent low SSTs, cold coastal upwelling, and short-lived El Niño events. A single marine molluscan faunal province presently extends from the northern limit of cold coastal upwelling (about 5°S) to the southern limit and beyond to south-central Chile. Long-term changes in thermal gradients from equatorial to temperate austral latitudes should have affected the distribution of molluscan species; latitudinal changes in fossil faunal distributions, therefore, might provide evidence for changing oceanographic conditions. The Pisco Basin of southern Peru has outcrops of Cenozoic marine sediments that flank centers of intense modern cold coastal upwelling. Sedimentological and paleontological evidence indicates that coastal upwelling with associated high primary and secondary productivity has been present since at least the late middle Eocene. A comparison of fossil molluscan faunas shows that coastal upwelling was not an effective biogeographic barrier between northernmost Peru and southern Peru during the late Eocene, when global SSTs were in decline but still high by Neogene standards. From the latest Oligocene to early middle Miocene, when ∂¹⁸O values were as low as they had been since the late Eocene and when many ‘tropical' molluscan taxa inhabited the coastal waters of Chile, the southern Peruvian margin with its coastal upwelling served as a selective two-way filter to molluscan migration, with no taxa from northern Peru or Chile present in both disjunct areas. During the late middle Miocene, when global SSTs were falling, several equatorial species expanded their ranges into the upwelling regime of southern Peru, but did not appear in Chile. During the late Miocene and early Pliocene, when global SSTs were high relative to the late Pliocene and Quaternary, several warm-water equatorial taxa were present in southern Peru and Chile and most taxa from southern Peru were also in Chile. These data suggest that the effectiveness of coastal upwelling as a biogeographic barrier and its characterization as ‘warm' depends on temperature contrasts with adjoining non-upwelling coastal waters.